The invention relates to a method and an apparatus for determining an operating state of a motor which is connected to a rigid network and has a rotation speed sensor.
The operating point of a motor connected to a rigid network is, for example, the mechanical shaft power which the motor emits to a machine that is being driven by it. For example, this mechanical shaft power which is emitted from the motor and the maximum permissible mechanical shaft power for the machine which is being driven by it can be compared in order to identify whether the machines that are being driven are in a correct state. The operating state may also comprise, for example, the actual rotation speed, the mechanical shaft power and further information.
A device and a method for correct electronic processing of operating data for an electrical motor are known from European Laid-Open Specification 0 240 684. The motor operating data include, for example, the rotation angle, the rotation speed, the rotation acceleration, the torque, the power, the thermal load, the motor voltage and the motor current. In this Laid-Open Specification, the motor current is monitored as the particularly indicative parameter of the load on the motor. This motor current is detected by means of a current sensor, for example a current transformer, in the supply line to the motor, is digitized, and is supplied to a central processor unit. This processor unit uses a program that is stored in the program memory to continually check the arriving actual values of the motor current for compliance with predetermined limit values and, furthermore, records switch-on, changeover and switch-off processes. The processor unit allocates to such selective operating data data relating to the absolute time of their occurrence, and stores such data in a data memory. The power supply for the device is designed such that it can operate autonomously and operationally reliably. This device is expediently associated solely with the electric motor for a lengthy time, preferably for its entire life or operating time, that is to say the device cannot be used simultaneously or successively for different motors, in order to obtain clear relationships for the reconstruction of the motor biography. Since the motor current is used here, this device is power-dependent. This means that the current transformer must be replaced if the motor current rises.
A universal circuit concept which comprises functional modules which can also be connected individually is known from the specialist report xe2x80x9cIntelligenter Motorschutzxe2x80x9d [Intelligent motor protection], by Dietrich Amft and Peter Waber, printed in the tenth International Symposium xe2x80x9cIndustrielle Automatisierungxe2x80x94automatisierte Antriebexe2x80x9d, [Industrial automationxe2x80x94automated drives], at the [Karl-Marx-Stadt Chemnitz] Technical University, from Feb. 14 to 16, 1989. Converter/transducer assemblies for detecting the motor supply line currents, analog/digital converters, an assembly for determining the motor temperature via winding sensors, a computer assembly for temperature calculation, a communication interface, a filter assembly for assessing current harmonics, a core-balance current transformer assembly, a function input, a display and a power supply assembly are provided as a functional module. The computer assembly has a single-chip microprocessor. The set current, actual current and temperature as well as coded faults can be indicated via the display. A warning is produced when the current actual value is greater than 80% to 100% of the rated value or, on the other hand, delayed tripping takes place if the rated value is exceeded.
An on-line diagnosis system for rotating electrical machines is known from EP 0 675 369 B 1, by means of which the state and the maintenance requirement for a machine are diagnosed. This system has a number of motor sensors which monitor different physical parameters and produce appropriate electrical signals. Signal converters/transducers convert the electrical signals to appropriate digital values. These values are collected by a processor, which compares these values, or a trend of the values, with predetermined basic values or trends. The processor then makes recommendations for the motor servicing interval, in order to ensure optimum performance and availability, with minimal costs and a minimal down time.
The invention is now based on the object of specifying a method and an apparatus for determining an operating state of a motor which is connected to a rigid network, which method is independent of power.
According to the invention, this object is achieved by the features of claim 1 and claim 12.
Since the phase voltages which are applied to the connecting terminals of the motor are evaluated rather than the current flowing in the motor in order to determine an operating state of a motor which is connected to a rigid network and has a rotation speed sensor, the method and the apparatus can be used for any motor, irrespective of the motor rating. The frequency and amplitude of these network voltages are determined and are used, in conjunction with a determined shaft rotation speed, the number of pole pairs in the motor and motor-specific parameters, to calculate the mechanical shaft power as an operating state. The phase voltages of the rigid network are not measured directly for this method, with the frequency and amplitude of these phase voltages which are applied to the connecting terminals of the motor being evaluated instead.
Since these phase voltages of the rigid network which are applied to the connecting terminals are only evaluated for the apparatus for determining an operating state, the apparatus does not require any voltage transformers. Since neither current nor voltage transformers are required for the apparatus, the design of this apparatus is independent of the rating of the associated motor. This apparatus can thus be used virtually independently of power.
The apparatus is subdivided into an analog evaluation circuit and a digital computation unit. The analog evaluation circuit is used to evaluate the frequency and the amplitude of the phase voltages of a rigid network which are applied to the connecting terminals of the motor. The digital computation unit uses the determined frequency and amplitude values, the measured shaft rotation speed, the number of pole pairs in the associated motor and motor-specific parameters to determine an operating state, for example the mechanical shaft power. Since the analog evaluation circuit has few components and the digital computation unit may preferably be one component, the wiring complexity is minimal. Furthermore, the physical volume that is occupied is minimal.
The inventive step is that the phase voltages of a rigid network which are applied to the connecting terminals of the motor are used for determining an operating state of a motor which is connected to this network. This means that current transformers are not used since their designs are related to power, which means that different apparatuses, which are subdivided into rating classes, must be provided for motors having different ratings.
The operating state of a motor is governed not only by a single value, for example the mechanical shaft power, but can also be composed of a number of values.
In one advantageous method, the time sequence of determined voltage sections of the phase voltages of the rigid network which are applied to connecting terminals of the motor is evaluated in order to determine the rotation direction of the network at the connecting terminals of the motor. The rotation direction of the network at the connecting terminals of the motor governs the rotation direction of the motor. Since the rotation direction can be determined from the phase voltages that are applied, a simple bar encoder or toothed wheel encoder, which cannot identify the rotation direction, is sufficient for use as the rotation speed sensor.
It is possible for a situation to arise in which one phase of the rigid network fails at the motor, for whatever reasons, so that a temperature sensor must be fitted to each winding, or must be inserted in it, for reliable protection of the winding of the motor against overtemperature, if no phase failure identification is provided. Phase failure identification allows the number of temperature sensors to be reduced to one, which is inserted into any of the motor windings.
In a further advantageous method, the time sequence of determined voltage sections of the phase voltages which are applied to connecting terminals of the motor is evaluated, and the amplitude of these voltage sections is evaluated, in order to determine a phase failure in the network.
In a further advantageous method, the measured temperature of the stator and a thermal model of the motor are used to determine the temperature of the rotor. Calculation of the temperature of the rotor means that it is now possible to use temperature-dependent parameters to correct the rotor resistance. The stator resistance can be used for this parameter in the same way, in which case the measured stator temperature is used directly in the correction calculation.
In one advantageous apparatus for determining an operating state of a motor, the analog evaluation circuit has a polyphase diode bridge, which is provided on the output side with an RC element and a device for suppressing any DC component. Furthermore, a high-value resistance is connected electrically in parallel with the RC element in order that a ripple signal derived from the rectified network voltage is produced at the output of the device for suppressing any DC component. The digital computation unit uses this ripple signal to determine the network frequency.
In a further advantageous embodiment of the apparatus for determining an operating state of a motor, a number of trigger circuits are used instead of the device for suppressing any DC component, and these are each connected on the input side to an input of the polyphase diode circuits, and on the output side to an input of the digital computation unit. The use of the trigger circuit considerably improves the frequency determination. Furthermore, this makes it easier to determine a phase failure, since the time sequence of the determined voltage drops across the diodes in the lower bridge arm of the polyphase diode bridge, and the amplitude of these determined voltage drops, are now evaluated.
In a further advantageous embodiment of the apparatus for determining an operating state of a motor, an optocoupler is used instead of each trigger circuit, and these optocouplers are in each case electrically connected in series with the diodes in the lower diode arm of the diode bridge. In this way, the current conduction times of the diodes in the lower diode arm of the diode bridge are determined with DC decoupling.
In one particularly advantageous embodiment, an electrical switch is connected electrically in series with the high-value resistor which is connected electrically in parallel with the RC element in the analog evaluation circuit, and this electrical switch is connected on the control side to the digital computation unit, with this connection having an optocoupler for DC isolation. This switch is used to connect the high-value resistor only at specific times. The power loss in this high-value resistor is reduced as a function of the duty ratio. If the switch is synchronized to the natural commutation times of the polyphase diode bridge, it is also possible to determine the network frequency, in addition to determining any phase failure. Furthermore, the power losses caused by the high-value resistor are then minimal. This pulsed high-value resistor can also be used in the other embodiments if there is an aim to design the apparatus for determining an operating state of a motor to have low losses.
Advantageous refinements of the apparatus according to the invention for determining an operating state of a motor which is connected to a rigid network are described in the dependent claims 12 to 30.